Transitions of Aggregation States for Concentrated Carbon Nanotube Dispersion

Because of the lack of appropriate techniques for the measurement of concentrated dispersions, dispersion states of carbon nanotube (CNT) dispersions have been evaluated for dilute dispersions by assuming the dispersion state being unchanged by dilution. In this paper, it is clarified that this assumption does not hold true at a high concentration region by a direct measurement of size distribution and anisotropy for CNT dispersions in a wide concentration region. CNT dispersions showed a dispersion-state transition as a form of rotation restriction at a certain concentration. In addition to this, CNT dispersions whose solutes have a large specific surface area showed another dispersion-state transition at a certain concentration as a form of aggregation growth. To prove these dispersion-state transitions from another point of view, the difference in sheet resistance of conducting layers made from different CNT dispersions coated on a glass substrate was investigated. It was confirmed that their sheet resistance also showed a clear difference. This difference can be explained from the viewpoint of dispersion-state transitions induced by the drying process

Multiscale Dynamics of Inhomogeneity-Free Polymer Gels

For precise understanding of the dynamics of gels, it is necessary to distinguish the effect of inherent cross-linking from accompanying inhomogeneity. This separation is realized by the use of inhomogeneity-free gel such as Tetra-PEG gel. We investigated the dynamics of Tetra-PEG gel by quasi-elastic scattering. Mesoscopic (length scale: ∼100 nm) motion was measured by dynamic light scattering (DLS). In addition to this scale, we used neutron spin echo (NSE) to measure microscopic (length scale: ∼1 nm) motion. From these measurements, it is revealed that the gels with no connectivity/topological inhomogeneities show the transition from Zimm mode to collective diffusion mode in larger length scale, even beyond the q-range of NSE. In addition to this, the absence of spatial inhomogeneities is reflected as disappearance of nondecay component in the intermediate dynamic structure factor. Through the combination analysis of DLS and NSE, the multiscale dynamics of gels is elucidated

SANS Studies on Spatial Inhomogeneities of Slide-Ring Gels

Slide-ring gels (SR gel) [previously termed as topological or polyrotaxane gels:  Okumura, Y.; Ito, K. Adv. Mater. 2001, 13, 485] have remarkable physical properties, such as large extensibility and mechanical strength. The SR gels are cross-linked polyrotaxane (PR) consisting of poly(ethylene glycol) (PEG) chains and α-cyclodextrin (CD), in which the cross-linkers are made of CD dimers and capable of sliding along the PEG chains. To elucidate the physical picture and properties, the scattering functions, I(q)s, of SR gel in NaOD aqueous solutions (NaODaq) and in deuterated dimethyl sulfoxide (d-DMSO) were investigated by small-angle neutron scattering (SANS) and were compared with those of pregel solutions, where q is the magnitude of the scattering vector. The following facts were disclosed: (1) The polyrotaxane chains take a rodlike conformation in d-DMSO, whereas a Gaussian chain in NaODaq. (2) The degree of inhomogeneities of SR gel in NaODaq has a minimum around the sol−gel transition, whereas that in d-DMSO increases monotonically with increasing cross-linker concentration. (3) I(q) of SR gel in NaODaq can be described by a Lorentz function, while that in d-DMSO is given by the sum of a squared Lorentz function and a scattering function for a rod. These differences in I(q) are ascribed to the difference in the stacking behavior of CD molecules on PEG chains in PR

Water-in-Ionic Liquid Microemulsion Formation in Solvent Mixture of Aprotic and Protic Imidazolium-Based Ionic Liquids

We report that water-in-ionic liquid microemulsions (MEs) are stably formed in an organic solvent-free system, i.e., a mixture of aprotic (aIL) and protic (pIL) imidazolium-based ionic liquids (ILs) containing the anionic surfactant dioctyl sulfosuccinate sodium salt (AOT). Structural investigations using dynamic light, small-angle X-ray, and small-angle neutron scatterings were performed for MEs formed in mixtures of aprotic 1-octyl-3-methylimidazolium ([C₈mIm⁺]) and protic 1-alkylimidazolium ([C_<i>n</i>ImH⁺], <i>n</i> = 4 or 8) IL with a common anion, bis­(tri­fluoro­methanesulfonyl)­amide ([TFSA^–]). It was found that the ME structure strongly depends on the mixing composition of the aIL/pIL in the medium. The ME size appreciably increases with increasing pIL content in both [C₈mIm⁺]­[TFSA^–]/[C₈ImH⁺]­[TFSA^–] and [C₈mIm⁺]­[TFSA^–]/[C₄ImH⁺]­[TFSA^–] mixtures. The size is larger for the <i>n</i> = 8 system than that for the <i>n</i> = 4 system. These results indicate that the shell part of MEs is composed of both AOT and pIL cation, and the ME size can be tuned by pIL content in the aIL/pIL mixtures

Kinetic Aspect on Gelation Mechanism of Tetra-PEG Hydrogel

We carried out a kinetic study on the gelation reaction of AB-type cross-end coupling of two tetra-arm poly­(ethylene glycol) (Tetra-PEG) prepolymers having amine (Tetra-PEG-NH2) and activated ester (Tetra-PEG-NHS) terminal groups by ATR-IR and UV spectroscopies. The reaction rate constant for the gelation of Tetra-PEG, kgel, was determined in aqueous solutions with varying both prepolymer volume fraction, ϕ, and molecular weight, Mw, of the prepolymers. It was clearly found that the value of kgel is independent of both ϕ and Mw, and is comparable to that of the corresponding linear-PEG system. The kgel value is obtained to be around 70 dm3 mol–1 s–1, which is much smaller than the reaction rates of typical diffusion-controlled reaction (e.g., 108–109 dm3 mol–1 s–1) and of cross-linking photopolymerization (104–105 dm3 mol–1 s–1). From these results, we concluded that the gelation reaction of Tetra-PEG gel is not diffusion-limited but reaction-limited process, i.e., the diffusion motion is much faster than the reaction rate. It is thus expected that Tetra-PEG prepolymer chains can diffuse in the solution during gelation process, leading to homogeneity and high-strength of Tetra-PEG gel. These discussions imply that, in order to achieve high-efficient and homogeneous gel, it is necessary to choose reaction groups so as to undergo reaction-limited reaction

Difference in Lower Critical Solution Temperature Behavior between Random Copolymers and a Homopolymer Having Solvatophilic and Solvatophobic Structures in an Ionic Liquid^†

The solubility and phase behavior of poly(benzyl methacrylate) (PBzMA) and poly(styrene-co-methyl methacrylate) (P(St-co-MMA)) in a hydrophobic ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethane sulfone)imide ([C2mim][NTf2]), have been explored as a function of temperature. Although both polymers have solvatophobic phenyl groups and solvatophilic methacrylate groups in the structure, their distribution on the polymer chains is quite different. In PBzMA, both structures are incorporated in each monomer unit, whereas in P(St-co-MMA)s the distribution is statistically determined by the monomer reactivity ratio of St and MMA. Both polymer solutions in [C2mim][NTf2] become turbid with an increase in temperature (lower critical solution temperature (LCST) behavior). The turbidity change occurs sharply at 100 °C for PBzMA, whereas it is sluggish for P(St-co-MMA)s. The LCST-type phase-separation temperature for P(St-co-MMA)s decreases with an increase of the St composition. The sluggish phase separation for P(St-co-MMA)s has been explained in terms of the presence of the MMA sequences along the polymer chain, which inhibits the St aggregation to a certain extent. The dynamic light scattering (DLS) measurements for PBzMA reveal that the hydrodynamic radius of PBzMA suddenly changes at 100 °C; below this temperature, no aggregation is observed. This result strongly implies that the coil-to-collapse transition is of the first-order type. It has been demonstrated that the LCST-type phase separation of the polymers in an ionic liquid is greatly affected by the distribution of the solvatophilic and solvatophobic groups on the polymer chains

Nearly Ideal Polymer Network Ion Gel Prepared in pH-Buffering Ionic Liquid

We report a high-toughness ion gel with a nearly ideal polymer network prepared in an imidazolium-based aprotic ionic liquid (aIL) with a controlled solution pH. We formed the ion gel from tetra-armed poly­(ethylene glycol) (TetraPEG), i.e., an A–B-type cross-end coupling reaction of two different TetraPEG prepolymers. To complete this A–B-type reaction, we needed to optimize the reaction rate such that the two TetraPEGs were mixed homogeneously, which strongly depends on the pH or [H⁺] in the aIL solution. To control solution pH, we established a “pH-buffering IL” by adding an imidazolium-based protic IL (as a proton source) and its conjugated base to the solvent aIL. We demonstrated that the pH-buffering IL exhibits a successful pH-buffering effect to maintain a constant pH (≈16.2, apparent value) during the gelation reaction. From a kinetic study, we found that the gelation reaction undergoes a simple second-order reaction of the two TetraPEGs in the pH-buffering IL. The gelation rate constant, <i>k</i>_gel, in the present ion gel system was 2 orders of magnitude smaller than that in the corresponding hydrogel system, which is ascribed to the difference in the activation entropy, Δ<i>S</i>^‡, of the cross-end coupling reactions. The reaction efficiency at the cross-linking point was experimentally estimated to be 92% by spectroscopic measurements. We thus conclude that a nearly ideal polymer network was formed in the pH-buffering IL system. This is reflected in the excellent mechanical property of the ion gel, even at a low polymer content (=6 wt %)

Phase Behavior of Block Copolymers in Selective Supercritical Solvent

The phase behavior of a block copolymer and supercritical fluid system was investigated. When a particular block was selectively swollen by the supercritical fluid, the apparent volume fraction of one domain was controlled by the pressure of the supercritical fluid. The morphological variation of polystyrene-<i>b</i>-poly­[2-(perfluoro­octylethyl) methacrylate]­s (PS–PFMAs) with different ratios of PS to PFMA and the total degree of polymerization were analyzed. Time-resolved small-angle neutron scattering measurement revealed that lamellar and hexagonal phases coexist metastably, which may be induced by fluctuation of the supercritical fluid